Notice in this example, the possessive adjective dein take the same ending as the indefinite article of ein from Schwester.

Explaination of indefinite Article: ein

In german, there are masc., fem, and neutral nouns.

You can tell by the article:
das, die, der

das is neutral

die is fem.

der is man

der corresponds to ---> ein
die corresponds to ---->eine (notice that fem. has extra "e" at the end)
das corresponds to --->ein

Mas. ein Computer
Fem. eine Lampe
Neuter ein Poster

A break down of the question:

Wie heißt deine Schwester (What is your sister's name.)

The word for sister is ==> die Schwester
-notice it is fem. article die
-die corresponds with eine

dein (your) is than changed to deine it's eine. That means adding a extra "e".

Why is it conjugated to Heißt in this question?
1)the Nomantive or the main subject in this sentence is the sister.
a)That means conjugating to the sie form or she form of heissen.
- adding "t" and dropping the "en"